For most themes:
Well depending on the theme you can try installing it by dragging and dropping the theme archive into the theme window.
You seem to have been trying to do this already and this process can fail if the theme creator packs other files or Dirs into the theme tar.

However if this does not work you may have to manually extract the themes tar to: /home/YourUserName/.themes/
(this answer by iRaveFuzion, new one below by Padster)
__________

The theme you want to install is a login theme for GDM. with the latest version of most distros, Ubuntu included, these themes aren't supported any more. Sorry.

How do you like Plasma 5? The best KDE Desktop ever. Definitely a nice improvement. Not decided yet. Haven't tried it yet. I do not like some of the changes. KDE is taking the wrong way. I am still sticking with KDE 3.5. I have no opinion, but wanted to vote anyway.

Copyright 2003-2015 KDE-Apps.org Team All rights reserved. KDE-Apps.org is not liable for any content or goods on this site.All contributors are responsible for the lawfulness of their uploads.KDE and K Desktop Environment are trademarks of KDE e.V.